Concern over the depletion and environmental impact of fossil fuels has stimulated strong interest in the development of alternative energy sources. Significant investments in areas such as batteries, fuel cells, hydrogen production and storage, biomass, wind power, algae, and solar energy have been made as society seeks to develop new ways of creating and storing energy in an economically competitive and environmentally benign fashion. The ultimate objective is to minimize society's reliance on fossil fuels and to do so in an economically competitive way that minimizes greenhouse gas production.
A number of experts have concluded that to avoid the serious consequences of global warming, it is necessary to maintain CO2 at levels of 550 ppm or less. To meet this target, based on current projections of world energy usage, the world will need 17 TW of carbon-free energy by the year 2050 and 33 TW by the year 2100. The estimated contribution of various carbon-free sources toward the year 2050 goal are summarized below:
Projected EnergySourceSupply (TW)Wind2-4Tidal2Hydro1.6Biofuels5-7Geothermal2-4Solar600Based on the expected supply of energy from the available carbon-free sources, many experts believe that solar energy is the most viable solution for reducing greenhouse emissions and alleviating the effects of global climate change.
Unless solar energy becomes cost competitive with fossil fuels, however, society will lack the motivation to eliminate its dependence on fossil fuels and will refrain from adopting solar energy on the scale necessary to meaningfully address global warming. As a result, current efforts in manufacturing are directed at reducing the unit cost (cost per kilowatt-hour) of energy produced by photovoltaic materials and products.
The general strategies for decreasing the unit cost of energy from photovoltaic products are (1) reducing process costs and (2) improving photovoltaic efficiency. Efforts at reducing process costs are directed to identifying low cost photovoltaic materials and increasing process speeds. Crystalline silicon is currently the dominant photovoltaic material because of its wide availability in bulk form. Crystalline silicon, however, possesses weak absorption of solar energy because it is an indirect gap material. As a result, photovoltaic modules made from crystalline silicon are thick, rigid and not amenable to lightweight, thin film products.
Materials with stronger absorption of the solar spectrum are under active development for photovoltaic products. Representative materials include CdS, CdSe, CdTe, ZnTe, CIGS (Cu—In—Ga—Se and related alloys), organic materials (including organic dyes), and TiO2. These materials offer the prospect of reduced material costs because their high solar absorption efficiency permits photovoltaic operation with thin films, thus reducing the volume of material needed to manufacture devices.
Amorphous silicon (and hydrogenated or fluorinated forms thereof) is another attractive photovoltaic material for lightweight, efficient, and flexible thin-film photovoltaic products. Stanford R. Ovshinsky was among the first to recognize the advantages of amorphous silicon (as well as amorphous germanium, amorphous alloys of silicon and germanium, including doped, hydrogenated and fluorinated versions thereof) as a photovoltaic material. S. R. Ovshinsky also recognized the underlying physical properties and practical benefits of the nanocrystalline, microcrystalline, and intermediate range order forms of silicon, germanium, silicon-germanium alloys and related materials. For representative contributions of S. R. Ovshinsky in the area of silicon-based photovoltaic materials see U.S. Pat. Nos. 4,217,374 (describing suitability of amorphous silicon and related materials as the active material in several semiconducting devices); 4,226,898 (demonstration of solar cells having multiple layers, including n- and p-doped); and 5,103,284 (deposition of nanocrystalline silicon and demonstration of advantages thereof); as well as his article entitled “The material basis of efficiency and stability in amorphous photovoltaics” (Solar Energy Materials and Solar Cells, vol. 32, p. 443-449 (1994)).
Approaches for increasing process speed and throughput include: (1) increasing the intrinsic deposition rates of the different materials and layers used to manufacture photovoltaic devices and (2) adopting a continuous, instead of a batch, manufacturing process. S. R. Ovshinsky has pioneered the automated and continuous manufacturing techniques needed to produce thin film, flexible large-area solar panels based on amorphous, nanocrystalline, microcrystalline, polycrystalline or composite materials. Although his work has emphasized the silicon and germanium systems, the manufacturing techniques that he has developed are universal to all material systems. Representative contributions of S. R. Ovshinsky to the field of high speed thin film manufacturing are included in U.S. Pat. Nos. 4,400,409 (describing a continuous manufacturing process for making thin film photovoltaic films and devices); 4,410,588 (describing an apparatus for the continuous manufacturing of thin film photovoltaic solar cells); 4,438,723 (describing an apparatus having multiple deposition chambers for the continuous manufacturing of multilayer photovoltaic devices); and 5,324,553 (microwave deposition of thin film photovoltaic materials).
A second general approach for decreasing the unit cost of energy from photovoltaic products is to improve photovoltaic efficiency. Photovoltaic efficiency depends on maximizing the generation of photoexcited charge carriers from a given amount of incident light and harvesting as many of the photoexcited charge carrier as possible. Both the intrinsic properties of the active photovoltaic material and the characteristics of the surrounding layers in the device structure are important for optimizing photovoltaic efficiency. As noted above, the absorption efficiency of the active photovoltaic material is critical in maximizing the number of photogenerated charge carriers. Once generated, however, the charge carriers must be able to migrate to the external contacts of the photovoltaic device to provide power to an external load. To maximize performance, it is necessary to recover the highest possible fraction of photogenerated carriers and to minimize losses in energy associated with transporting photogenerated carriers to the outer contacts. It is especially important that transport of charge carriers occurs without recombination in order to insure high photovoltaic efficiency. The presence of defects in a photovoltaic material degrades photovoltaic efficiency by providing sites for recombination of photogenerated charge carriers.
One problem with current plasma-deposited photovoltaic materials is the presence of a high concentration of intrinsic defects in the as-deposited state. The intrinsic defects include structural defects (e.g. dangling bonds, strained bonds, unpassivated surface states, non-tetrahedral bonding distortions, coordinatively unsaturated silicon or germanium) that create electronic states within the bandgap of the photovoltaic material. The midgap states detract from solar conversion efficiency by acting as nonradiative recombination centers that deplete the concentration of free carriers generated by absorbed sunlight. Instead of being available for external current, the energy of many of the photoexcited free carriers is dissipated thermally through nonradiative decay. The external current delivered by a photovoltaic material is reduced accordingly.
It has been observed that the concentration of intrinsic defects increases as the deposition rate of amorphous silicon-based photovoltaic increases. In order to improve photovoltaic efficiency, it has been necessary in the prior art to reduce the speed of plasma deposition. It is believed that reduced deposition rates suppress the formation of defects in the as-deposited material or provide sufficient time to allow as-formed defects to equilibrate to a more regular bonding configuration.
A need exists in the art for a method for preparing thin film photovoltaic materials (including amorphous, nanocrystalline, microcrystalline, and polycrystalline forms of silicon, germanium, and alloys of either) at high deposition rates without sacrificing photovoltaic efficiency due to recombination processes associated with intrinsic defects. The low deposition rates needed to achieve high photovoltaic efficiency limit the economic competiveness of conventional plasma deposition processes. It is desirable to develop new deposition processes that produce photovoltaic materials, especially silicon-containing materials of any crystalline or non-crystalline form, at high deposition rates without compromising quality.